1. Field of the Invention
The present invention relates generally to a linear positioning system and more specifically to a stabilizing means for a linear positioning system such as a ball screw actuator.
2. Description of the Prior Art
A variety of systems are known in the prior art and utilized for linearly moving and positioning a workpiece relative to a housing. Some of such positioning systems use an elongated housing in the form of an elongated hollow cylinder with a slot therein and a piston or other motion transfer member moveable reciprocally along the cylinder. In these systems, a carrier bracket is connected to the piston or other motion transfer member and extends through the slot for the purpose of transferring the linear movement of the piston or motion transfer member to a workpiece. In some embodiments, movement of the piston or other motion transfer member results from the use of piston rods or pneumatic pressure selectively delivered to pneumatic pressure chambers within the cylinder on opposite sides of the piston.
In other embodiments, movement of the piston or other motion transfer member is in the form of a spindle nut having internal threads matching the external threads of an elongated, rotatably driven threaded spindle. In these embodiments, the rotatably driven spindle extends substantially from one end of the cylinder to the other and the spindle nut which is rotationally fixed relative to the cylinder, advances or retracts linearly along the cylinder in response to rotational movement of the spindle. Linear positioning systems of this type are sometimes referred to as threaded actuators or ball screw actuators.
By rotating the externally threaded spindle, the internally threaded spindle or drive nut, and thus the connected workpiece, can be linearly positioned along the length of the cylinder or other supporting housing. In actuators of this type, efficiencies of cost and operation are achieved by decreasing the diameter of the threaded spindle and increasing the speed at which the spindle rotates, thereby increasing the speed at which the drive nut and thus the workpiece can be moved from one position to another. Unfortunately, both of these tendencies place significant limitations on the actuator. Because the threaded spindle is supported only at its ends, the middle portion of the spindle will tend to sag, thereby creating an out-of-balance or eccentric condition when the spindle is rotated. This out-of-balance condition is accentuated with a decrease in the diameter of the spindle, an increase in the rotational speed of the spindle or an increase in the length of the spindle. Thus, for a given spindle diameter, an unacceptable increase in the rotational speed or spindle length will cause the spindle to wobble or whip in jump rope fashion along the axis of the cylinder. This not only places an upper limit on the acceptable linear speed and spindle length, but also leads to premature fatigue and wear of the spindle and drive nut and faulting of the rotary drive mechanism. Such vibrations and whipping action can also impair the accuracy of the positioning system.
In ball screw actuators as described above, the critical rotation speed at which the spindle begins to unacceptably vibrate or whip is a function of the spindle length and the spindle diameter. Although the critical speed can be increased by using larger spindle diameter for a particular spindle length, this leads to increased cost. Further, as the diameter, and thus the mass of inertia, of the spindle increases, larger and more expensive rotational drive motors are needed.
A variety of solutions have been proposed to dampen the vibrations or whipping action of the threaded spindle and thus effectively increase the critical speed at which a given spindle can be rotated. These solutions function primarily by providing a vibration damping member on the spindle between the support points. Such clamping members are generally moveable or displaceable along the spindle as the spindle nut advances and retracts along the length of the spindle. Some of these prior means are disclosed in U.S. Pat. No. 4,794,810 issued to Parsons, U.S. Pat. No. 4,821,456 issued to Nogaki, U.S. Pat. No. 4,878,390 issued to Hauser, U.S. Pat. No. 5,251,501 issued to Katahira, U.S. Pat. No. 5,425,607 issued to Hardesty and U.S. Pat. No. 5,531,557 issued to Springer.
Although some of these function to increase the critical speed for a given actuator system to some extent, various drawbacks and limitations continue to exist. These include the complexity of the devices and the limit to which the critical speed can be increased, among others. Accordingly, there is a continuing need in the art for a stabilizer mechanism usable with a ball screw actuator or the like which limits the vibration or whipping action of the spindle and thus increases the critical speed at which a given spindle rotates.
The present invention relates to a stabilizing mechanism for a ball screw actuator or the like which dramatically increases the critical speed for a given actuator system of up to three times or more. More specifically, the stabilizing mechanism of the present invention includes one or more a support members which are moveable linearly along the rotating spindle to selected locations between the support points to provide support for the spindle. This reduces vibrations and whipping action and thus increases the critical speed of the spindle.
Still more specifically, the stabilizing mechanism of the present invention is designed for use with a ball screw actuator or the like having an elongated housing in the form of a hollow cylinder, a threaded spindle extending the length of the cylinder and a spindle or drive nut rotationally fixed relative to the housing for linear movement along the cylinder. The stabilizing mechanism generally includes one or more stabilizing units, each of which includes a stabilizer nut with internal threads substantially matching those of the spindle and means in the form of a stop for limiting the linear movement of the stabilizer nut relative to the spindle. In the preferred embodiment, a bearing member is provided to limit friction between such elements.
Each stabilizing unit in accordance with the present invention further includes a first member or slider which is moveable linearly relative to the housing and which engages the stop positioned on the housing. Mounted within the slider is a bearing member which is adapted for engagement by the stabilizer nut. In the preferred embodiment, the slider is rotationally fixed, but linearly moveable, relative to the housing.
In a most preferred embodiment, the stabilizer unit in accordance with the present invention includes a pair of linearly spaced bearings mounted within the slider, with the stabilizer nut positioned therebetween. In a still further preferred embodiment, grease or other drag inducing means is provided between an external surface of the stabilizer nut and an internal surface of the slider to create a controlled drag between the slider and the stabilizer nut to advance the stabilizer nut in a controlled movement along the spindle.
Accordingly, it is an object of the present invention to provide an improved stabilizer mechanism for a ball screw actuator or the like.
Another object of the present invention is to provide a stabilizer mechanism for a ball screw actuator or the like which effectively increases the critical speed at which a given actuator can operate.
Another object of the present invention is to provide a stabilizing means for a linear positioning system of the type having a rotatable spindle which includes a stabilizer nut with internal threads substantially matching that of the spindle.
A further object of the present invention is to provide a stabilizing mechanism for a ball screw actuator or the like which includes an outer slider and an inner stabilizing nut with controlled drag means disposed between such elements.
A still further object of the present invention is to provide a stabilizing mechanism for a ball screw actuator or the like which provides a spindle damping mechanism moveable along the spindle and positioned between the points of spindle support.